For memory devices and for numerous other applications, bi-stable devices or circuits are used. For example, for storing one bit of information in a memory, a bi-stable device can be used which is switchable between at least two different and persistent states. When writing a logical “1” into the device, it is driven into one of the two persistent states and when writing a logical “0”, or erasing the logical “1”, the device is driven into the other of the two different states. Each of the states persists until a next step of writing information into the device or erasing information in the device proceeds.
Flash erasable programmable read only memory (FEPROM, also referred to as flash memory) is used in semiconductor devices and provides for rapid block erase operations. Typically, flash memory only uses one transistor per memory cell versus the two transistors per memory cell for known electrically erasable programmable read only memory (EEPROM). Thus, flash memory takes up less space on a semiconductor device and is less expensive to produce than EEPROM. Nevertheless, the development of further space-saving components of semiconductor devices and cost-efficient fabrication techniques for producing such devices continues.
To that end, the use of materials with bi-stable electrical resistance for semiconductor device applications has been studied. The resistance states of the material can be changed reversibly by applying appropriate electrical signals to the material. These electrical signals should be larger than a given threshold VT and longer than a given time t. The resistance state of the material can be read or analysed by applying other signals which are non-destructive to the conductivity state if they are much smaller than VT.
Transition-metal oxides are one class of materials that can be conditioned such that they exhibit the desired bi-stable electrical resistance. Non-volatile two-terminal memory devices based on transition-metal oxides have been disclosed. Such devices comprise at least one memory cell, which comprises the arrangement of at least two electrodes being provided in contact with a transition-metal oxide layer. Depending on the polarity of electrical pulses applied to one of the electrodes relative to the other electrode, the electrical resistance of the transition-metal oxide switches reversibly between at least two different and persistent states. An example of such a device is given in U.S. Pat. No. 6,815,744.
The conditioning process that the transition-metal oxides are subjected to in order that the switching between the resistance states may be done comprises subjecting the transition-metal oxide to an appropriate electrical signal for a sufficient period of time, this being done via electrical signals applied to the electrodes contacting the transition-metal oxide layer as discussed above. The conditioning process generates a confined conductive region in the transition-metal oxide that can be reversibly switched between two or more resistance states.
Drawbacks of the above-described devices are associated with the conditioning process. This is because, not only is the conditioning process time-consuming, it is required per cell incorporated in such a device. Furthermore, the confined conductive region that is generated by the conditioning process occurs at an arbitrary position in the dielectric material, i.e., the position of the conducting path is not controllable by well-defined process parameters. This may cause a large variation in the electrical properties of such devices, that are otherwise nominally identical, to be observed. In combination, these issues pose severe drawbacks for the use of memory cells based on transition-metal oxides in a production-type array.
Accordingly, it is desirable to provide a programmable resistance memory cell that mitigates and/or obviates the drawbacks associated to known programmable resistance memory cells.